Both commercial and miliary aircraft typically carry auxiliary power units (APU) and often additionally may utilize a so-called emergency power unit (EPU). In some instances, the functions of both are combined.
In emergency systems, EPUs, or APUs that operate additionally as EPUs must be brought into full operational capacity in a relatively short period of time, such as two or three seconds. In the usual case, these systems employ a turbine wheel for driving emergency power sources such as an electrical generator, a hydraulic pump or both so as to provide the energy necessary to continue to operate the aircraft. Consequently, it is necessary that the turbine wheel be accelerated up to normal operating speed in a relatively short period of time so that if an APU is being utilized to provide emergency power, it can reach a self sustaining speed. Where an EPU is being utilized, it still must be accelerated rapidly and then its operation maintained for some predetermined time period.
Because these systems are intended for operation in emergency conditions, it is necessary that they have an extremely high degree of reliability. One obviously necessary feature is the ability to start rapidly without fail and maintain operation for the desired time period.
As noted above, these systems typically employ a turbine wheel, or even a complete turbine engine to drive the emergency power sources. Thus, in order that there be reliable starting of the system, ignition of fuel in a combustor feeding the turbine wheel must be reliably had and the continued combustion of the fuel maintained over the desired period of operation.
As is typical with many turbine systems, the control of fuel flow is achieved by a so-called .DELTA.P valve. .DELTA.P valves conventionally include a variable orifice along with a valve that is operable, in response to command from a fuel control system, to control the pressure drop across the orifice. Thus, for a given pressure drop, a constant fuel flow will be assured.
These valves typically respond to increases or decreases in pressure downstream of the valve as is well-known. As a consequence, in some cases, the pressure rise within a combustor associated with a turbine wheel to be driven and fueled by fuel via the .DELTA.P valve will result in a lesser pressure drop across the .DELTA.P valve which in turn means a reduction in fuel flow until the valve responds. Consequently, a lean flameout may result immediately after ignition where valve response to the pressure surge of ignition is slow, and intolerable situation in a piece of equipment adapted for emergency use.
The present invention is directed to overcoming one or more of the above problems.